Telecommunications access systems provide for voice, data, and multimedia transport and control between the central office (CO) of the telecommunications service provider and the subscriber (customer) premises. Prior to the mid-1970s, the subscriber was provided phone lines (e.g., voice frequency (VF) pairs) directly from the Class 5 switching equipment located in the central office of the telephone company. In the late 1970s, digital loop carrier (DLC) equipment was added to the telecommunications access architecture. The DLC equipment provided an analog phone interface, voice CODEC, digital data multiplexing, transmission interface, and control and alarm remotely from the central office to cabinets located within business and residential locations for approximately 100 to 2000 phone line interfaces. This distributed access architecture greatly reduced line lengths to the subscriber and resulted in significant savings in both wire installation and maintenance. The reduced line lengths also improved communication performance on the line provided to the subscriber.
By the late 1980s, the limitations of data modem connections over voice frequency (VF) pairs were becoming obvious to both subscribers and telecommunications service providers. ISDN (Integrated Services Digital Network) was introduced to provide universal 128 kbps service in the access network. The subscriber interface is based on 64 kbps digitization of the VF pair for digital multiplexing into high speed digital transmission streams (e.g. t TI/T3 lines in North America t E1/E3 lines in Europe). ISDN was a logical extension of the digital network that had evolved throughout the 1980s. The rollout of ISDN in Europe was highly successful. However, the rollout in the United States was not successful t due in part to artificially high tariff costs which greatly inhibited the acceptance of ISDN.
More recently, the explosion of the Internet and deregulation of the telecommunications industry have brought about a broadband revolution characterized by greatly increased demands for both voice and data services and greatly reduced costs due to technological innovation and intense competition in the telecommunications marketplace. To meet these demands, high speed DSL (digital subscriber line) modems and cable modems have been developed and introduced. The DLC architecture was extended to provide remote distributed deployment at the neighborhood cabinet level using DSL access multiplexer (DSLAM) equipment. The increased data rates provided to the subscriber resulted in upgrade DLC/DSLAM transmission interfaces from T1/E1 interfaces (1.5/2.0 Mbps) to high speed DS3 and OC3 interfaces. In a similar fashion, the entire telecommunications network backbone has undergone and is undergoing continuous upgrade to wideband optical transmission and switching equipment.
Similarly, wireless access systems have been developed and deployed to provide broadband access to both commercial and residential subscriber premises. Initially, the market for wireless access systems was driven by rural radiotelephony deployed solely to meet the universal service requirements imposed by government (i.e., the local telephone company is required to serve all subscribers regardless of the cost to install service). The cost of providing a wired connection to a small percentage of rural subscribers was high enough to justify the development and expense of small-capacity wireless local loop (WLL) systems.
Deregulation of the local telephone market in the United States (e.g., Telecommunications Act of 1996) and in other countries shifted the focus of fixed wireless access (FWA) systems deployment from rural access to competitive local access in more urbanized areas. In addition, the age and inaccessibility of much of the older wired telephone infrastructure makes FWA systems a cost-effective alternative to installing new, wired infrastructure. Also, it is more economically feasible to install FWA systems in developing countries where the market penetration is limited (i.e., the number and density of users who can afford to pay for services is limited to small percentage of the population) and the rollout of wired infrastructure cannot be performed profitably. In either case, broad acceptance of FWA systems requires that the voice and data quality of FWA systems must meet or exceed the performance of wired infrastructure.
Wireless access systems must address a number of unique operational and technical issues including:
1) Relatively high hit error rates (BER) compared to wire line or optical systems; and
2) Transparent operation with network protocols and protocol time constraints for the following protocols:                a) ATM;        b) Class 5 switch interfaces (domestic GR-303 and international VS.2);        c) TCP/IP with quality-of-service QoS for voice over IP (VoIP) (i.e., RTP) and other H.323 media services;        d) Distribution of synchronization of network time out to the subscribers;        
3) Increased use of voice, video and/or media compression and concentration of active traffic over the air interface to conserve bandwidth;
4) Switching and routing within the access system to distribute signals from the central office to multiple remote cell sites containing multiple cell sectors and one or more frequencies of operation per sector; and
5) Remote support and debugging of the subscriber equipment, including remote software upgrade and provisioning.
Unlike physical optical or wire systems that operate at bit error rates (BER) of 10-11, wireless access systems have time varying channels that typically provide bit error rates of 10-3 to 10-6. The wireless physical (PHY) layer interface and the media access control (MAC) layer interface must provide modulation, error correction and ARQ protocol that can detect and, where required, correct or retransmit corrupted data so that the interfaces at the network and at the subscriber site operate at wire line bit error rates.
Wireless access systems, as well as other systems which employ a shared communications media, must also provide a mechanism for allocating available communications bandwidth among multiple transmitting and receiving groups. Many wireless systems employ either a time division duplex (TDD) time division multiple access (TDMA) or a frequency diversity duplex (FDD) frequency division multiple access (FDMA) allocation scheme illustrated by the timing diagram of FIGS. 3A and 3B. TDD 300 shares a single radio frequency (RF) channel F1 between the base and subscriber, allocating time slices between the downlink 301 (transmission from the base to the subscriber) and the uplink 302 (transmission from the subscriber to the base). FDD 310 employs two frequencies F1 and F2, each dedicated to either the downlink 311 or the uplink 312 and separated by a duplex spacing 313.
For wireless access systems which provide Internet access in addition to or in lieu of voice communications, data and other Web based applications dominate the traffic load and connections within the system. Data access is inherently asymmetric, exhibiting typical downlink-to-uplink ratios of between 4:1 and 14:1.
TDD systems, in which the guard point (the time at which changeover from the downlink 301 to the uplink 302 occurs) within a frame may be shifted to alter the bandwidth allocation between the downlink 301 and the uplink 302, have inherent advantages for data asymmetry and efficient use of spectrum in providing broadband wireless access. TDD systems exhibit 40% to 90% greater spectral efficiency for asymmetric data communications than FDD systems, and also support shifting of power and modulation complexity from the subscriber unit to the base to lower subscriber equipment costs.
Within the spectrum allocated to multi-channel multipoint distribution systems (MMDS), however, some spectrum is regulated for only FDD operation. Since the total spectrum allocated to MMDS is relatively small (2.5-2.7 GHz, or about 30 6 MHz channels), some service providers may desire to utilize the FDD-only spectrum, preferably utilizing the TDD-based equipment employed in other portions of the MMDS spectrum.
There is, therefore, a need in the art for enabling TDD-based equipment to operate utilizing frequencies reserved for FDD only operation.
In one embodiment the present disclosure provides a method of TDD operation in a first subscriber unit having a cable modem. The method includes receiving in the first subscriber unit a first signal from a first base station on a downlink frequency during a first time period. The method includes transmitting from the first subscriber unit a second signal to the first base station on an uplink frequency during a second time period following the first time period. The downlink frequency and the uplink frequency are separated by a predefined duplex spacing.
In another embodiment, the present disclosure provides a cable modem configured to use TDD in a first subscriber station. The modem includes a receiver circuit to receive a first signal on a first frequency designated for downlink transmission during a first time period. The modem also includes a transmitter circuit to transmit a second signal on a second frequency different from the first frequency and designated for uplink transmission during a second time period following the first time period. The first frequency is employed for downlink transmission to a second subscriber station during the second time period and the second frequency is employed for uplink transmission from the second subscriber station during the first time period.
In still another embodiment, the present disclosure provides a method of time sharing frequencies reserved for FDD operation for use in conjunction with a cable modem. The method includes receiving in the cable modem a first signal from a first base station on a downlink frequency during a first time period. The method also includes transmitting from the cable modem a second signal to the first base station on an uplink frequency during a second time period following the first time period, wherein the downlink frequency and the uplink frequency are separated by a predefined duplex spacing.
In a further embodiment, the present disclosure provides a cable modem configured to use TDD. The cable modem includes a transmitter circuit configured to transmit a first signal on a first frequency to a first subscriber station during a first time period and transmit a second signal on a second frequency, different from the first frequency, to a second subscriber station during a second time period following the first time period. The cable modem also includes a receiver circuit configured to receive a third signal on the second frequency from the second subscriber station during the first time period and receive a fourth signal on the first frequency from the first subscriber during the second time period.